Answer:
Thyroglossal duct cysts are ectodermal remnants which may develop along the line of descent of the thyroid gland in the neck from the base of the tongue. During the 4th to 7th week of gestation, the foramen cecum at the base of the tongue is the site of development of the thyroid diverticulum which descends in the anterior midline of the neck. It maintains its connection to the foramen cecum via the thyroglossal duct. Normally, the duct is obliterated once the thyroid gland reaches its final destination. however, persistence of the duct leads to the development of cysts and sinuses which may be located anywhere along the line of descent of the thyroid gland. Typically, however, the cysts are below the level of the hyoid bone and in the midline. Thyroglossal duct cysts never have a primary external opening because embryologically the tract does not reach the surface of the neck. However, external fistulous drainage may be observed with infection of the cysts. Ectopic thyroid tissue is identified within the thyroglossal duct remnant in about 25-35% of cases, and papillary adenocarcinoma has been described in up to 10% of patients undergoing thyroglossal duct excision in adulthood. Rarely, complete failure of the thyroid gland to descend results in a lingual thyroid which may present as a mass at the base of the tongue. Or the gland may only partially descend resulting in a sublingual thyroid. The thyroglossal duct cyst should be distinguished from these entities to prevent inadvertent removal of what may be the patient's only thyroid tissue.
The key diagnostic features of these neck lumps are their midline position and movement on tongue protrusion and swallowing. They present most commonly in toddlers and older children as a painless neck lump. Most are intimately related to the hyoid bone, which explains their relation to the tongue and muscles of swallowing. About 10 percent arise under the chin as submental swellings and may be confused with midline dermoid cysts. Secondary infection of thyroglossal cysts may occur, and their congenital origin is only suspected when recurrence or persistent drainage follows surgical incision.
All patients should have a serum TSH level. If the level is normal and the cyst is infrahyoid then further work-up is usually not necessary. If otherwise, a radioisotope scan should be performed. If the cyst is located cephalic to the hyoid bone, further imaging with MR or CT should also be performed.
The operation of choice was described by Sistrunk in 1920. A transverse incision is made through the skin and platysma which are reflected. Fistulized cysts should be excised with a transverse elliptical incision encompassing the fistula. The sternohyoid and thyrohyoid muscles are divided and the cyst in continuity with a central segment of hyoid bone and a core of suprahyoid tissue up to the base of the tongue is dissected and removed. Attempts should not be made to isolate the duct itself since it is usually noncontinuous and no definitive dissection plane can be identified. Infected cysts should be treated with warm soaks, antibiotics, and if necessary incision and drainage prior to complete excision after resolution of the infection. Recurrence rates of 5 to 7% for primary excisions have been reported.